1. Field of the Invention
The present invention relates to a method used in a communication device in wireless communication system, and more particularly, to a method of remapping HARQ timeline in TDD configuration change.
2. Description of the Prior Art
Time division duplex (TDD) offers flexible deployments without requiring a pair of spectrum resources. Currently, LTE TDD allows for asymmetric uplink-downlink (UL-DL) subframe allocations by providing seven different TDD UL-DL configurations. The TDD UL-DL configuration may or may not match the instantaneous traffic situation. Thus, the current mechanism provides TDD UL-DL reconfiguration for traffic adaptation. It has been shown that the average cell throughput can be improved to a large extent by allowing traffic adaptation in LTE TDD system.
Hybrid automatic repeat request (HARQ) is a transmission technique widely adopted in modern wireless communication systems. HARQ operates by retransmitting an identical copy of the original transmission or another redundancy version upon transmission error. The receiver then combines the previously corrupted transmissions with the retransmitted one. In LTE TDD systems, the timing relation between the feedback information indicating a transmission error and the corresponding retransmission are separately and differently defined for each of the seven configurations due to the different allocations of the UL-DL subframes (referring to FIG. 1, which illustrates a schematic diagram of HARQ timelines of TDD UL-DL configurations in the LTE system).
However, the applicant notices a problem of HARQ timeline mismatch due to the TDD reconfiguration since the HARQ timeline is dependent on the adopted TDD UL-DL configuration. In other words, dynamic switching among different TDD UL-DL configurations gives rise to HARQ timeline mismatches, especially for the UL case due to the constraint of synchronous HARQ operations. Such a mismatch problem could severely affect the transmission performance and degrade the achievable performance gain brought by the dynamic TDD traffic adaptation.
Please refer to FIG. 1, the HARQ timing relations are separately and differently defined for each of the seven TDD UL-DL configurations. In the case of TDD reconfiguration, the HARQ timelines for the UL HARQ processes, i.e, the UL grant, PUSCH, and PHICH, could be interrupted due to the different numbers of DL/UL subframes and/or different allocations of the DL/UL subframes. For example, there are four UL HARQ processes in TDD UL-DL configuration #1 while there are only three UL HARQ processes in TDD UL-DL configuration #3. The problem of HARQ timeline mismatch thus arises when the system switches from TDD UL-DL configuration #1 to configuration #3, and vice versa.
Take examples associated to the HARQ timeline mismatch problem. Please refer to FIGS. 2A-2C and 3, which illustrate schematic diagrams of HARQ timeline mismatch due to TDD UL-DL reconfiguration. In FIG. 2A, a UE configured with TDD UL-DL configuration #1 in radio frame #0 is reconfigured with TDD UL-DL configuration #2 in radio frame #1. The UE received UL grant in subframe #9 of TDD UL-DL configuration #1 in radio frame #0 shall transmit a PUSCH in subframe #3 in radio frame #1 based on the HARQ timeline defined in TDD UL-DL configuration #1. However, the PUSCH transmission is mismatched since subframe #3 of TDD UL-DL configuration #2 in radio frame #1 is a DL subframe. In FIG. 2B, if a PUSCH transmission in subframe #3 of TDD UL-DL configuration #2 in radio frame #1 is successful, a UL grant shall be allocated in subframe #8 in radio frame #0 based on the HARQ timeline defined in TDD UL-DL configuration #2. However, it is impossible that the UE receives the UL grant in subframe #8 of TDD UL-DL configuration #1 in radio frame #0 since subframe #8 of TDD UL-DL configuration #1 in radio frame #0 is a UL subframe. In FIG. 2C, the UE transmits PUSCH in subframe #3 of TDD UL-DL configuration #1 in radio frame #0 and then receives PHICH/NACK in subframe #9 of TDD UL-DL configuration #1 in radio frame #0. Based on the HARQ timeline of TDD UL-DL configuration #1, the UE shall transmit PUSCH in subframe #3 of TDD UL-DL configuration #1 in radio frame #1. However, the PUSCH is mismatched since subframe #3 of TDD UL-DL configuration #2 in radio frame #1 is a DL subframe.
Similarly, in FIG. 3, the UE configured with a carrier with TDD UL-DL configuration #1 in radio frame #0 is reconfigured with TDD UL-DL configuration #3 in radio frame #1. The UE receives UL grant in subframe #4 of TDD UL-DL configuration #1 in radio frame #0, and then transmits PUSCH in subframe #8 of TDD UL-DL configuration #1 in radio frame #0. Based on the HARQ timeline of TDD UL-DL configuration #1, the UE shall receive PHICH in subframe #4 in radio frame #1. However, it is impossible that the UE receives PHICH in subframe #4 of TDD UL-DL configuration #2 in radio frame #1 since subframe #4 of TDD UL-DL configuration #2 in radio frame #1 is a UL subframe.
Therefore, there is a need to solve the HARQ timeline mismatch problems.